If you want to update an existing project, update the dependencies in the
composer.json file according to the versions found in the composer.json file
from the Symfony Standard Edition and then follow the upgrade instructions
(for Symfony.

If you are only using some of the Symfony components, just update the versions
of these components in your composer.json file.

There are a few additional things I want to mention about the Symfony Standard
Edition.

The Symfony Standard Edition now only depends on MIT/BSD licensed
libraries and bundles (Swiftmailer
recently changed its license to MIT). If you are using the
JMSDiExtraBundle or JMSSecurityExtraBundle bundles, you can still
use them by manually updating your composer file (see the commit
that removed them).

The Symfony Standard Edition repository does not come with a
composer.lock file anymore. This was mainly done because Composer
installs a different version of the symfony/Icu component depending on
your PHP configuration. But anyway, as the Standard Edition is a generic
application, it is better to not lock out some specific versions.